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Genetic Crosses Review

Genetic Crosses Review. Do Now: Do the dihybrid cross on your notes sheet. Dihybrid cross: . RW Rw rW rw. Phenotype Ratio: 9:3:3:1. RW Rw rW rw. 5 Types of Genetic Crosses. The dihybrid cross is one of the 5 types of genetic crosses we have studied.

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Genetic Crosses Review

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  1. Genetic Crosses Review Do Now: Do the dihybrid cross on your notes sheet.

  2. Dihybrid cross: RW RwrWrw • Phenotype Ratio: 9:3:3:1 RW Rw rWrw

  3. 5 Types of Genetic Crosses • The dihybrid cross is one of the 5 types of genetic crosses we have studied. • In a dihybrid cross, the inheritance of 2 traits is predicted at the same time • Since each parent has 4 possible combinations of alleles, dihybrid crosses are 4 x 4 squares.

  4. Complete dominance • Complete dominance is the first type we looked at. • One allele hides the effect of the other. • AA = dominant phenotype • Aa = dominant phenotype • aa = recessive phenotype • Possible crosses include: • Heterozygous x heterozygous (Aa x Aa) • 3:1 phenotype ratio, 1:2:1 genotype ratio • Homozygous x heterozygous (aa x Aa and AA x aa) • 1:1 phenotype and genotype ratios • Homozygous x homozygous (AA x AA, aa x aa, and AA x aa) • 100% have the same genotype and phenotype

  5. Complete Dominance Example • In humans, albinism (a) is recessive to normal pigmentation (A). If an albino man has a child with a woman who is heterozygous for the trait, what is the probability their child will be an albino?

  6. Codominance • For codominant traits, BOTH alleles show up in the phenotype • AA = dominant phenotype • Aa = both phenotypes (i.e. red + white fur) • aa = recessive phenotype • Possible crosses include: • Heterozygous x heterozygous (Aa x Aa) • 1:2:1 phenotype ratio, 1:2:1 genotype ratio • Homozygous x heterozygous (aa x Aa and AA x aa) • 1:1 phenotype and genotype ratios • Homozygous x homozygous (AA x AA, aa x aa, and AA x aa) • 100% have the same genotype and phenotype

  7. Codominance Example • In a species of tropical frog, there are frogs that have blue skin, frogs that have black skin, and frogs that have both black and blue skin. • If two black and blue frogs mate, what will the phenotype ratio of their offspring be?

  8. InComplete dominance • Traits with incomplete dominance produce a heterozygous phenotype that is a blend of the two allele’s effect • AA = dominant phenotype • Aa = blended phenotype (pink flowers • aa = recessive phenotype • Possible crosses include: • Heterozygous x heterozygous (Aa x Aa) • 1:2:1 phenotype ratio, 1:2:1 genotype ratio • Homozygous x heterozygous (aa x Aa and AA x aa) • 1:1 phenotype and genotype ratios • Homozygous x homozygous (AA x AA, aa x aa, and AA x aa) • 100% have the same genotype and phenotype

  9. Incomplete dominance example • A type of pepper comes in three colors, red, orange, and yellow. If a gardener crosses two orange pepper plants, what will the phenotype ratio of the offspring be?

  10. Sex-linked Traits • The alleles for sex linked traits are located on the sex chromosomes (X chromosome in humans) • Recessive sex-linked traits are much more common in males than females, since they have only 1 copy

  11. Sex-Linked traits In a Punnett square for a sex-linked trait, the sex chromosomes are used instead of letters. X+X+ normal f X+Xo carrier f XoXo color-blind f X+Y normal m XoY color-blind m

  12. Sex-Linked Example • Because she has a brother with hemophilia, a biology student wanted to figure out the probability that she was a carrier for the X-linked disorder. Neither of her parents has hemophila. What is the probability she is a carrier of the trait?

  13. QUIZ Monday! • There will be a quiz on genetic crosses this Monday! Each of the 5 types will show up on it. • Do your homework for practice!

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